This invention relates to an optical viewing device that permits a person to simultaneously view a scene and also view a superimposed display of other information and, more particularly, to such a device in the form of a head-mounted display with a wide field of view and low observability.
The ready availability of a wide variety of information is becoming increasingly important to combat soldiers, law-enforcement personnel, and others exposed to potentially dangerous situations. Head-mounted displays have been developed to allow the wearer to view a primary scene directly, and simultaneously to view a display of the same or another scene, a map, and/or tactical data. In general, such displays include a line of sight to the primary scene, an image source for the display of the other information, and a beam combiner that mixes the display image from the image source into the direct view of the primary scene.
There are several technical challenges to the implementation of such systems. One such challenge is achieving good display image intensity in a wide range of daylight and darkness situations, while keeping the weight of the head-mounted device acceptably low. Another is providing a wide field of view of the primary scene. If the field of view is too small, the advantages of the availability of the additional information are outweighed by the reduced visibility of the primary scene caused by overlay occlusion.
Yet another problem with such head-mounted displays used by soldiers, law-enforcement personnel, and others in combat situations, as compared with those who are not in combat, is that the head-mounted display must have a low observability to opponents. If some significant amount of the light generated by the display escapes from the device in the forward direction, that escaped light may allow the opponent to locate and target the user of the device, especially in low-light-level conditions. It is therefore important that no substantial amount of light escapes from the device in a forward direction so as to be visible to the opponent.
While head-mounted displays are known, there is a need for a device that combines the features of a wide field of view, good scene and display image intensity to the user, and low observability to opponents. The present invention fulfills this need, and further provides related advantages.
The present invention provides a situation awareness viewing device, preferably in the form of a head-mounted display, enabling a wide field of view (on the order of 40 degrees in the inventors"" prototype), good scene and display image intensity for the user, and low observability in the forward direction away from the user. The polarizing performance of the device is nearly independent of incident wavelength in the visible range, so that it may be used with a color image source. The device is light in weight and has good optical performance and convenience for the user. It is also compact, with a prototype being designed to fit within a 1-inch cube.
In accordance with the invention, a situation awareness viewing device includes a polarizing beam splitter comprising a cube of a material transparent to light and having an index of refraction greater than 1, and a wire grid polarizer lying within the cube on a cube-diagonal plane extending between two diagonally opposed edges of the cube. The material transparent to light is preferably transparent to visible light, and most preferably is a plastic. The polarizing beam splitter has a first optical axis extending from a first face of the cube toward an opposing second face of the cube and lying at an angle of 45 degrees to the cube-diagonal plane, and a second optical axis extending from a third face of the cube toward an opposing fourth face of the cube and lying at an angle of 45 degrees to the cube-diagonal plane. The second optical axis is perpendicular to the first optical axis. The device further includes an external polarizer external to the cube on the first optical axis and disposed to intercept light incident upon the first face along the first optical axis, an image source external to the cube and disposed to send a display image incident upon the third face along the second optical axis, a quarter-wave plate external to the cube and disposed to intercept a beam of light traveling along the second optical axis and passing out of the cube through the fourth face, and a mirror external to the cube and disposed to reflect light passing through the quarter-wave plate back through the quarter-wave plate and to the fourth face. The mirror preferably is a spherical concave mirror. These components are desirably supported in a head mount.
In another embodiment, a situation awareness viewing device includes a polarizing beam splitter comprising a wire grid polarizer that may or may not lie within a solid cube of a material transparent to light and having an index of refraction greater than 1. The polarizing beam splitter has a first optical axis lying at an angle of 45 degrees to the wire grid polarizer, and a second optical axis lying at an angle of 45 degrees to the wire grid polarizer and perpendicular to the first optical axis. An external polarizer lies on the first optical axis on a first side of the wire grid polarizer, an image source lies on the second optical axis on the first side of the wire grid polarizer disposed to send a display image toward the wire grid polarizer along the second optical axis, a polarization direction changing device lies on the second optical axis on a second side of the wire grid polarizer, and a mirror lies on the second optical axis such that the polarization direction changing device is between the mirror and the wire grid polarizer. The mirror is disposed to reflect light traveling along the second optical axis back through the polarization direction changing device. In both embodiments, the some or all of the elements may be laminated or bonded together so that there are no air gaps therebetween, or the elements may be spaced apart with air gaps therebetween.
The use of the wire grid polarizer in the present design yields important benefits in this application that are not achieved with other polarizers or by the wire grid polarizer in other applications. The wire grid polarizer utilizes metallic stripes to achieve the polarization, not a dielectric stack. The wire grid polarizer has a high reflectivity for s-polarized light and a high transmission of p-polarized light, with the s- and p-polarization defined relative to the wire grid polarizer features as will be discussed subsequently. It may therefore be used with an unpolarized light image source, an important advantage because unpolarized light image sources are available with much higher brightnesses than the available polarized light image sources. In some prior head-mounted displays, the brightness of the polarized image source was so low that a mechanism had to be provided to darken the image of the primary scene so as not to obscure the image of the image source, adding to the weight, size, and complexity of the device. That is not necessary with the present approach.
The wire grid polarizer retains its polarizing capability even for incident light at a relatively high angle of incidence. The result is that the device has a wide field of view. The wire grid polarizer has low net attenuation of the light from the primary scene and from the display image source, so that the user views on the order of 30 percent or more of the initial light intensity of the primary scene and the projected display image. The wire grid polarizer transmits p-polarized light from the image source and reflects s-polarized light in the forward direction away from the user. However, the external polarizer intercepts and blocks this reflected s-polarized light so that very little if any light escapes from the situation awareness viewing device.
The present approach allows the use of an unpolarized image source, which has a higher image intensity than a polarized image source such as a plane-polarized liquid crystal display. If the plane-polarized image source is used, the polarized emitted light that is incident upon the polarizing beam splitter must be p-polarized so that it may pass through the polarizing beam splitter. None of this p-polarized energy may be reflected by the polarizing beam splitter outwardly from the device, so no polarizing device comparable with the p-oriented polarizer of the present approach is needed to achieve low observability. On the other hand, the p-polarized liquid crystal display has a relatively low image intensity and therefore cannot satisfactorily serve as the image source in a situation awareness viewing device.
The situation awareness viewing device permits operation at night where little or no background light precludes the user from seeing the world. In this case, the subject display system may be used to project a generic night vision-generated scene into the user""s wide field of view head mounted display, with ancillary data superimposed onto the night vision scene. The projected night vision image may or may not be boresighted to the user""s line of sight.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.